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Full text of "Handbook Of Chemical Engineering - I"

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126
CHEMICAL ENGINEERING
The surface of the valve seat is determined by the strength of the material. F = bj where F = surface of seat, square inches, p = pressure on the back of the valve, pounds per square inch; A = area of the valve on which p is acting, square inches, and b = bearing pressure of valve on seat, pounds per square inch = 10,000 Ib. for steel, 2,000 for red brass, bronze or gun metal, 2,500-3,000 for phosphor bronze, 1,000 for cast iron, 750 for leather and 375 for rubber. For high-speed pumps these values should be reduced to allow for shocks.
The spring pressure produced under the valve, which is the spring load in pounds, divided by the area of the valve A, may be 1 per cent of the working pressure, for the discharge valve, with a maximum of 5 Ib. per square inch. For the suction valve the spring pressure should be limited to 0.5 Ib. per square inch. If the suction comes in under a head heavier springs should be used.
Figure 22 shows a leather disc valve, Fig. 23 a conical wing valve, hi = h cos a, where hi = lift of valve, h = free opening measured perpendicular to the seat, a =
FIG. 22.—Leather-faced wing valve.
FIG. 23.—Conical wing valve.
angle of valve; for a — 45°, hi = 1.41A, or valve must lift 41 per cent higher than a flat-seated valve to give the same opening. Figure 24 shows a clapper valve, Fig. 25 a leather-faced clapper valve and Fig. 26 a ball valve, used for thick liquids.
Air Chambers.—The obj ect of an air chamber is to provide an elastic elemen t in the pipe line and produce a uniform flow.  Simple pumps require large air cham-

FIG. 24.—Bronze clapper valve.
FIG. 25.—Leather-faced clapper valve.
FIG. 26.—Ball valve.
bers (eight times the displacement of the plunger per stroke), duplex pumps only small air chambers (four times the displacement), duplex low service (75 Ib. pressure) and small general service pumps require no air chamber. They are a necessity on all crank and flywheel and power pumps. If the air coming in with the water is not sufficient to replenish that absorbed by the discharge water an air charging device should be used. This may be an independent air compressor or simply a pipe connected to one of the pump chambers, and provided with two check valves for air inlet and outlet. On pumps for over 300-lb. pressure, where the air is absorbed rapidly by the water a spring-loaded ram, called an alleviator should be used, Fig. 27.
A vacuum chamber performs the same duty on the suction pipe and should have its inlet on the dead end of a tee so as to be in line with the current, Fig. 28. It should be placed on the suction pipe of every good-sized pump and is a necessity on high lifts and pumps producing an irregular flow. When the water carries much air the vacuum chamber may act as an air collector from